A Group III nitride semiconductor formed on a substrate has been heretofore used as a functional material for fabricating a Group III nitride semiconductor light-emitting device with a pn-junction type structure capable of emitting visible light at a short wavelength, such as a light-emitting diode (LED) and a laser diode (LD) (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-332364). For example, at the fabrication of an LED emitting near ultraviolet light, blue light or green light, aluminum gallium nitride (AlXGaYN, wherein 0≦X, Y≦1 and X+Y=1) is formed on a substrate to a thickness of several μm (hereinafter referred to as an “underlying layer”) and works to improve the crystallinity and at the same time, facilitate the taking-out of light. Also, this semiconductor is utilized for constituting an n-type or p-type clad layer (see, for example, Kokai No. 2003-229645). On the other hand, gallium indium nitride (GaYInZN, wherein 0≦Y, Z≦1 and X+Z=1) can be utilized for constituting a light-emitting layer (see, for example, Japanese Examined Patent Publication (Kokoku) No. 55-3834).
In the conventional Group III nitride semiconductor light-emitting device, the underlying layer is generally gallium nitride (GaN) formed through a GaN or AlN buffer layer, or aluminum gallium nitride (AlXGaYN, wherein 0≦X, Y≦1 and X+Y=1) formed through a buffer layer, comprising AlGaN having an intermediate lattice constant, between the substrate and the underlying layer. In this case, the buffer layer plays the role of relieving the strain between the substrate and the underlying layer and aids in terminating a dislocation immediately after the underlying layer starts growing, thereby suppressing the propagation of a dislocation into the upper part of the underlying layer. Furthermore, the buffer layer reduces the propagation of a dislocation into the light-emitting layer, and the strain due to difference in the lattice constant, so as to give a small difference of lattice constant compared to a light-emitting layer comprising GaYInZN (wherein 0≦Y, Z≦1 and Y+Z=1) or the like.
However, even in the underlying layer where the propagation of a dislocation is suppressed by the buffer layer, the dislocation density can be as large as about 1×109 cm−2, and a reduction in dislocations is indispensable for enhancing the properties of a Group III nitride semiconductor light-emitting device. In this respect, studies have been made on the forming conditions of the buffer layer and the forming method of the underlying layer.
With respect to the method for this purpose, there have been heretofore proposed a method of applying a process of imparting irregularities to the substrate (see, for example, Kokai No. 2000-331947), forming an insulating film on the substrate (see, for example, Kokai No. 2002-16001), processing a stacked structure grown on the substrate (see, for example, Kokai No. 2004-35275) or forming an insulating film on a deposit (see, for example, Kokai No. 2002-289527) and, by utilizing the shape obtained, promoting the crystal growth in the transverse direction to terminate the dislocation. However, in such a method, as the substrate or a deposit layer on the substrate is processed, the formation of an insulating film or a patterning process such as photolithography and etching is complicated and the finally formed film allows for the presence of a dislocation distribution corresponding to the film shape.
Furthermore, a processing method of previously performing a pit-forming treatment before the epitaxial growth is sometimes employed (see, for example, Kokai No. 2003-124128 and Kokai No. 2002-261027). However, even in such a method, the process is cumbersome because the substrate, on which a film is grown, is once taken out from the growth furnace and, after applying a processing for the pit formation in the grown film, a layer capable of exerting a device function, such as light-emitting layer, is again grown.
As for the method of not processing the substrate or a deposit layer thereon, for example, pits are formed in the underlying layer and then filled up so as to cause change in the dislocation direction and terminate the dislocation, such as a method of growing a p-type doping layer or an n-type doping layer on irregularities generated on an undoped underlying layer, thereby flattening the irregularities (see, for example, Kokai No. 2000-353821 and Kokai No. 2000-357820), or a method of once stopping the growth to fill the pits up (see, for example, Kokai No. 2002-367908).
On the other hand, with respect to the pit generated in a Group III nitride semiconductor, it is known that pits are formed not only by selecting the growing conditions such as temperature and pressure but also by doping an impurity at a high concentration (see, for example, Japan Journal of Applied Physics, Vol. 31, pp. 2883-2888 (1992)). As for the method utilizing this, a method of doping Si at a fairly high concentration to form pits is known (see, for example, Kokai No. 2004-47764).
In the present invention, this formation of pits by the addition of an impurity is employed and a Group III nitride semiconductor stacked structure with a small dislocation density is obtained by controlling the kind of the impurity element added, the concentration of the impurity added, and the layer structure.